1. Field of the Invention
The present invention relates to a neuro element that models a neuron of the human being, which assigns weights individually to input signals inputted from a plurality of input terminals, inputs the respective weighted input signals to an arithmetic unit and performs an operation, and, when the result of the operation exceeds a predetermined value, fires and outputs a predetermined output signal. The present invention also relates to an integrated circuit apparatus for realizing such a neuro element.
2. Description of Related Art
A neuro element is a device that models a neuron, which forms a tissue such as the brain and eyes of the human being, by means of an electronic circuit. More specifically, by replacing one cell body, one axon and a plurality of dendrites and synapses of a neuron with equivalent electronic parts or electronic circuits, the neuro element is realized.
FIG. 1 is a schematic view for explaining the concept of the neuro element. A plurality of input signals with voltages V1 through Vn, inputted from a plurality of input terminals I1 through In, are weighted by weighting factors W1 through Wn, respectively, and the neuro element performs an operation on these weighted input signals, for example, the operation of addition as follows.                     θ        =                  ∑                                    V              i                        ⁢                          W              i                                                              =                                            V              1                        ⁢                          W              1                                +                                    V              2                        ⁢                          W              2                                +                                    V              3                        ⁢                          W              3                                +                      …            ⁢                                                   ⁢                          V              n                        ⁢                          W              n                                          Then, when the value of θ that is the result of this operation exceeds a predetermined threshold value TH, an output signal with a predetermined voltage Vout is outputted, and this signal becomes an input signal to the neuro element in the next stage.
FIG. 2 is a circuit diagram of a conventional ordinary neuro element. In this example, by substituting a plurality of fixed resistors 13-1 through 13-n for the synapses, an arithmetic circuit 12 for the cell body, and wires or the like for the dendrites and axons, a neuron of a human being is modeled as a whole. Input signals (whose voltages are V1 through Vn) from a plurality of input terminals I1 through In are weighted by resistances R1 through Rn of the respective fixed resistors 13-1 through 13-n. Accordingly, the input voltages V1 through Vn are converted into currents with magnitudes V1/R1, V2/R2, V3/R3, . . . , Vn/Rn and arithmetically summed together, and then inputted into the arithmetic circuit 12 composed of, for example, a comparator, etc. Then, when the sum of the input currents to the arithmetic circuit 12 exceeds a predetermined threshold value, an output signal with a predetermined voltage Vout is outputted from the arithmetic circuit 12 and supplied as an input signal to the neuro element in the next stage. Note that, regarding the specific structure of the arithmetic circuit 12, it is possible to use various circuits other than the circuit configuration shown in FIG. 2.
By the way, a plurality of the neuro elements as mentioned above are combined together and used for fields, such as character recognition and voice recognition, where general digital computers are weak. However, in the case of realizing a neuro element modeling a characteristic of the neuron of the human being, that is, the function of gradually speeding the reaction, i.e., making the reaction more sensitive, by receiving the same stimulation repeatedly many times, for example, it is necessary to change the weighting factor stepwise, according to the accumulated number of times of inputting (applying) the input signal corresponding to a stimulation received by the human being, and holds the weighting factor changed stepwise in a nonvolatile manner.
As a neuro element capable of changing the weighting factor stepwise and holding the weighting factor changed stepwise in a nonvolatile manner, there is a known invention which is disclosed in Japanese Patent Application Laid-Open No. 6-21531 (1994), and constructs a resistor functioning as the weighting means shown in FIG. 2 with a variable resistor using a chalcogen compound.
However, in the neuro element of the above-mentioned invention disclosed in Japanese Patent Application Laid-Open No. 6-21531 (1994), although the weighting factor can be changed according to the magnitude of a pulse voltage applied to the chalcogen compound used as the weighting means (variable resistor), it is impossible to change the weighting factor according to the number of times the pulse voltage was applied.
The reason for this is that the change in the resistance of the chalcogen compound uses a phase transition between a crystalline state and an amorphous state due to Joule heat generated by the current, and therefore the amplitude of the applied pulse voltage is reflected on the occupancy ratio between the crystal region and amorphous region to be formed, and then stored. However, the information about the previous input pulse voltage (the occupancy ratio between the crystal region and the amorphous region formed by the previous input pulse voltage), which was stored just before this moment in the weighting means made of the chalcogen compound, is overwritten by the information about the newly applied pulse voltage. In other words, the information stored in the weighting means made of the chalcogen compound corresponds only to the latest applied pulse voltage.
Hence, in the neuro element of the above-mentioned invention disclosed in Japanese Patent Application Laid-Open No. 6-21531 (1994), there is no change between the resistance of the chalcogen compound after application of one input signal and the resistance after application of the same input voltage again. In order to simplify the explanation, let consider the neuro element with one input. For example, suppose that, with the application of a pulse voltage having the amplitude of 3 V, the occupancy ratio between the crystal region and the amorphous region of the chalcogen compound was brought into a state produced by the application of the pulse voltage with the amplitude of 3 V, and the resistance changed from 100 kΩ to 10 kΩ. Thereafter, when the same pulse voltage with the amplitude of 3 V is applied again, the occupancy ratio between the crystal region and the amorphous region of the chalcogen compound can be brought only into the state obtained by the application of the pulse voltage with the amplitude of 3 V. Consequently, even when the application of the pulse voltage with the amplitude of 3 V is repeated many times, the resistance of the chalcogen compound does not change and remains 10 kΩ.
Hence, in the neuro element of the above-mentioned invention disclosed in Japanese Patent Application Laid-Open No. 6-21531 (1994), when the same input signal is inputted repeatedly, the resistance of the chalcogen compound functioning as the weighting means does not change. Therefore, in order to fire the neuro element, it is necessary to apply a pulse voltage with larger amplitude sufficient to fire the neuro element. Thus, this conventional neuro element will never fire by continuing the application of the same input signal. For the above-mentioned reasons, the neuro element of the invention disclosed in Japanese Patent Application Laid-Open No. 6-21531 (1994) is not able to model the function of the neuron of the human being which reacts more sensitively by receiving the same stimulation repeatedly many times.
Additionally, FIG. 22 of Japanese Patent Application Laid-Open No. 6-21531 (1994) mentioned above illustrates a neuro element comprising a plurality of transistors and weighting means using, for example, SRAM cells, capacitors and floating gates, connected to the gates of the transistors. However, such a neuro element suffers from a problem of an increase in the area occupied by the weighting means on the circuit.